The invention relates to a method and apparatus for detecting malfunction such as clogging or aging of a flowmeter including at least one transducer, and serving also to generate a conditioned signal based on an analog signal from said transducer.
The invention is applicable in measurement systems where accuracy depends on a magnitude which, although not metrological, must remain within a certain range in order to ensure that the measurement system operates properly. By way of example, in the field of measuring the flow rate of a fluid such as gas or water, for example, ultrasound flowmeters can be used. Such flowmeters generally comprise two ultrasound transducers disposed in a flow of fluid. In use, the transducers alternate between acting as an emitter and as a receiver. In order to measure the propagation time of a sound wave between the two transducers, one known method consists in exciting the emitter transducer with an excitation pulse. That pulse causes the emitter transducer to emit an ultrasound wave into the medium between the two transducers. The wave propagates towards the receiver transducer. The method consists in detecting the first oscillation of said wave on its arrival at the receiver transducer. The propagation time is then the time between the instant at which the emitter transducer is subjected to the excitation pulse and the instant at which the first oscillation of the wave reaching the receiver transducer is detected. The method consists in detecting the first oscillation of the wave by detecting when a voltage threshold is crossed. That method makes it necessary firstly to detect very low voltage levels, and secondly to have accurate control over the trigger threshold of the device for detecting the arrival of an oscillation so as to avoid introducing a delay in measuring the propagation time. When propagating through a fluid that is flowing, ultrasound waves take different lengths of time to propagate between the two transducers respectively in the upstream direction and in the downstream direction, and the flow rate of the fluid can be calculated from this difference. The two transducers are associated with an electronic circuit. The circuit controls the transducers and analyzes the analog signals delivered by the receiver transducer. One such device is described in greater detail in patent EP 0 426 309. Although the amplitude of the analog signal output by the receiver transducer is not a parameter that is required for computing flow rate, this magnitude must nevertheless present some minimum value in order to ensure that the electronic system associated with the transducers operates properly and in order to guarantee some minimum level of accuracy in flow rate measurement.
A problem that is frequently encountered with flowmeters of that type is that they become clogged up by particles entrained in the flowing fluid. In particular, these particles deposit on all of the xe2x80x9chydraulicxe2x80x9d portions of the flowmeter, for example on the active surfaces of the transducers and/or on the mirrors for modifying the path followed by waves within the fluid. Such clogging inevitably attenuates the waves that are transmitted and thus decreases the amplitude of the signal delivered by the receiver transducer. When clogging becomes extreme, the electronic system malfunctions since it is no longer able to process the analog signal output by the receiver transducer. Until now, this problem has been solved by dismantling and verifying the amount of clogging inside the flowmeter after some determined length of time. Naturally, such a solution presents a significant maintenance cost and is not satisfactory insofar as flowmeter clogging depends on the quality and the type of the impurities present in the flowing fluid.
An object of the invention is to mitigate those drawbacks by implementing a method and a device for detecting malfunction such as clogging or aging in an ultrasound flowmeter, said flowmeter having at least one transducer, and said method also making it possible to generate a conditioned signal on the basis of an analog signal coming from said transducer. Such a method and device make it possible to warn the user or the maintenance team that it is necessary to clean the flowmeter if it is becoming clogged or to replace it if it is aging.
Another object of the invention is to servo-control certain parameters of the electronic system in order to increase the operating range of said electronic system.
In the invention, these objects are achieved by a method comprising the following steps:
measuring the receive signal VIN output by the transducer; and
comparing a characteristic of the receive signal with a predetermined reference characteristic VREF;
said method being characterized in that it comprises the following additional steps:
storing a peak voltage VPK of the receive signal VIN;
generating an alarm signal VAL when a trigger characteristic VDEC of the receive signal VIN is less than the predetermined reference characteristic VREF;
defining a threshold voltage VTH proportional to the peak amplitude VPK of the receive signal in such a manner that VTH=Kxc3x97VPK, K being a factor depending on the transducer;
comparing the receive signal VIN with the threshold voltage VTH; and
generating a conditioned output signal VOUT in a first state when the receive signal VIN is greater than the threshold voltage VTH, and in a second state when the receive signal VIN is less than the threshold voltage VTH.
In a first variant implementation, the reference characteristic VREF is a voltage, and the trigger characteristic VDEC is the peak voltage VPK of the receive signal VIN.
In a second variant implementation, the reference characteristic VREF is a derivative of voltage, and the trigger characteristic VDEC is a derivative of the peak voltage VPK of the receive signal VIN.
An advantage of this method of detecting malfunction lies in the fact that the receive signal VIN output by the transducer is used simultaneously for generating the alarm signal VAL, the conditioned output signal VOUT, and for defining the threshold voltage VTH.
The device comprises:
a transducer delivering a receive signal VIN; and
a conditioning circuit (1) for conditioning the receive signal and comprising an input IN connected to the transducer, and an output OUT delivering a conditioned output signal VOUT;
the conditioning circuit comprising:
a selector (10) having its input connected to the input IN, and receiving the value of the predetermined reference voltage VREF, said selector delivering at its output a threshold voltage VTH that is servo-controlled to the receive signal VIN, and at its output AL a malfunction detection signal VAL whenever the peak amplitude of the receive signal VPK is below a predetermined reference voltage VREF; and
a comparator (20) having a first input connected to the input IN receiving the receive signal VIN and a second input connected to the selector receiving the threshold voltage VTH, an output of the comparator constituting the output OUT of the conditioning circuit generating a conditioned output signal VOUT having a first state when the amplitude of the receive signal is greater than the value of the threshold voltage, and a second state when the amplitude of the receive signal is less than the value of the threshold voltage VTh.
Thus, by using a modulatable threshold voltage VTH it is possible to extend the operating range of the electronics significantly relative to the amplitude of the receive signals and compared with using a fixed comparison threshold.
In addition, the servo-control performed in this way on the threshold voltage VTH enables propagation time measurements to be made from the second or third oscillation of the receive signal, and that cannot be envisaged with a fixed threshold that is not servo-controlled to the peak voltage VPK.
By way of example, the output signal AL is in a second state providing the entire measurement system is operating correctly. As soon as a malfunction is detected, the output signal AL switches into a first state corresponding to issuing the signal VAL indicating that malfunction has been detected. Alternatively, the malfunction detection signal can be a pulse, or a succession of pulses emitted over a determined length of time.
The reference voltage VREF is initially selected to be equal to a first reference voltage VREF1. The reference voltage VREF1 is selected in such a manner that the alarm signal is generated before the transducer ceases to deliver any receive signal. Thereafter, i.e. as soon as a first alarm signal VAL has been generated, the reference voltage VREF is modified and is selected to be equal to a second reference voltage VREF2, the second reference voltage VREF2 being lower than the first reference voltage VREF1 so that a second alarm signal is generated when the transducer ceases to deliver any receive signal.
Thus, using a modulatable reference voltage VREF makes it possible to issue an alarm signal before the signal output by the transducer has become completely unusable. Furthermore, the receive signal remains sufficient for the device as a whole to continue operating until the second alarm is issued, while allowing for the necessary measures to be taken to repair, clean, or change the measuring device.